The "AC" button is an "off switch" rather than the other way round as in previous models.

You will find you need to "activate off" every time you remove the keys from the ignition (if you don't want the AC on).

The idea being is to keep the interior at a correct "filtered climate level" which helps keeps the windows clear during adverse weather conditions.

Just another feature that MB decided you should have.

My W210 E240 and W209 CLK 240 had the same "feature".